Recently, hydrophobic or superhydrophobic surfaces have attracted a great deal of attention in many industrial areas due to the surface properties that make them potentially useful in various applications, such as optics, automobiles, building materials, and electronics.
Surface morphology is an important factor that influences the hydrophobicity or wettability of a surface. Cassie's law is used to describe the effective contact angle of a liquid on a composite surface and explains how roughing up a surface increases the apparent surface angle. The law is stated as in Formula I:cos θ′=f cos θ−(1−f)where θ′ represents the apparent contact angle on a rough surface, θ represents the intrinsic contact angle on a flat surface, f represents the fraction of the solid/water interface, and (1−f) represents the fraction of the air/water interface. A contact angle is the angle at which a liquid/vapor interface meets the solid surface. A contact angle of 0° denotes complete wettability and no drop formation, whereas a contact angle of 180° denotes complete unwettability. Cassie's law implies that with a small f and a large θ it is possible to create surfaces with a very large contact angle, i.e., with a reduced contact area. A composite with a rough surface has a water repelling quality, i.e., is hydrophobic.
Surface energy is another factor that affects the hydrophobicity of a surface. In general, a surface with high surface energy is likely to be hydrophilic, whereas a surface with low surface energy is likely to be hydrophobic. Thus, treatments that are capable of reducing the surface energy of a composite may increase the hydrophobicity of the composite surface.